To provide automobile engines with improved fuel efficiency and higher performance for environmental protection, so-called downsizing for reducing engine displacement by 20-50% is recently accelerated. Also, direct-injection engines are combined with turbochargers to increase compression ratios. Improvement in the efficiency of engines inevitably results in higher engine temperatures, which may cause power-decreasing knocking. Accordingly, improvement in the coolability of parts particularly around valves has become necessary.
As a means for improving coolability, Patent Reference 1 discloses a method for producing an engine valve comprising sealing metal sodium (Na) in a hollow portion of a hollow valve stem. With respect to a valve seat, Patent Reference 2 teaches a method for directly buildup-welding a valve seat on a cylinder head of an aluminum (Al) alloy by high-density heating energy such as laser beams to improve the coolability of a valve, which is called “laser cladding method.” As an alloy for buildup-welding the valve seat, Patent Reference 2 teaches a dispersion-strengthened Cu-based alloy comprising boride and silicide particles of Fe—Ni dispersed in a copper (Cu)-based matrix, Sn and/or Zn being dissolved in primary Cu-based crystals.
The valve temperature during the operation of an engine is about 150° C. lower in the above metal-sodium-filled valve (valve temperature: about 600° C.) than in a solid valve, and the Cu-based alloy valve seat produced by the laser cladding method lowers the temperature (about 700° C.) of a solid valve by about 50° C., preventing knocking. However, the metal-sodium-filled valves suffer such a high production cost that they are not used widely except some vehicles. The Cu-based alloy valve seats produced by the laser cladding method, which do not contain hard particles, have insufficient wear resistance, suffering seizure by impact wear. Also, the direct buildup-welding on cylinder heads needs the drastic change of cylinder head production lines and large facility investment.
With respect to a valve seat press-fit into a cylinder head, Patent Reference 3 discloses a two-layer structure comprising a valve-abutting layer formed by Cu powder or Cu-containing powder (sintered iron alloy layer containing 7-17% of Cu) and a valve seat body layer (sintered iron alloy layer containing 7-20% of Cu) for improving thermal conduction, and Patent Reference 4 discloses a sintered Fe-based alloy having porosity of 10-20% by dispersed hard particles, which is impregnated with Cu or its alloy.
Further, Patent Reference 5 discloses a sintered Cu-based alloy valve seat, in which hard particles are dispersed in a dispersion-hardened Cu-based alloy having excellent thermal conductivity. Specifically, a starting powder mixture comprising 50-90% by weight of Cu-containing matrix powder and 10-50% by weight of a powdery Mo-containing alloy additive, the Cu-containing matrix powder being Al2O3-dispersion-hardened Cu powder, and the powdery Mo-containing alloy additive comprising 28-32% by weight of Mo, 9-11% by weight of Cr, and 2.5-3.5% by weight of Si, the balance being Co.
However, the Cu content of at most about 20% in Patent References 3 and 4 fails to sufficiently improve the thermal conductivity. Though Patent Reference 5 teaches that Al2O3-dispersion-hardened Cu powder can be produced by heat-treating Cu—Al alloy powder atomized from a Cu—Al alloy melt in an oxidizing atmosphere for selective oxidation of Al, there is actually limit of increasing the purity of an Al2O3-dispersed Cu matrix formed from an Al-dissolved Cu—Al alloy. The inclusion of more hard particles (for example, 40-50% by weight) increases attackability to a valve, a mating member, and the inclusion of less hard particles (for example, 10-20% by weight) deteriorates the deformation resistance and wear resistance of the valve seat, resulting in remarkably contradictory tendency with respect to the amount of hard particles.